The present invention relates to a process for producing polyethylene in particular having improved mechanical properties.
Polyethylene is known for use in the manufacture of a wide variety of articles. The polyethylene polymerisation process can be varied in a number of respects to produce a wide variety of resultant polyethylene resins having different physical properties which render the various resins suitable for use in different applications. In particular, it is known to use polyethylene for use in applications where the polyethylene is required to have crack resistance, both resistance to rapid and to slow crack growth. For example, polyethylene is known for use in the manufacture of pipes where it is required that the material of the pipe has sufficient crack resistance so as to avoid inadvertent fracture in service. Polyethylene is also used for blow moulding and for the manufacture of films. For such applications the polyethylene is also required to have good mechanical properties, such as impact resistance, tear strength and environmental stress cracking resistance (ESCR).
Chromium-based catalysts used for the production of polyethylene have been known for some time. Typically, for the manufacture of polyethylene used as a pipe resin, the polyethylene is polymerised in a reactor using a chromium-based catalyst to product a monomodal molecular weight distribution of the resultant polyethylene resin.
It is known in the art that the physical properties, in particular the mechanical properties, of a polyethylene product can vary depending on what catalytic system was employed to make the polyethylene. This is because different catalyst systems tend to yield different molecular weight distributions in the polyethylene produced.
EP-A-O647661 discloses chromium catalyst compositions and polymerisation processes therewith. The catalyst compositions comprise at least two chromium catalyst systems which have different supports having different pore volumes and surface areas. It is disclosed that the resins produced are useful for film applications. The specification does not address the problem of the production of polyethylene having crack resistance for use in the manufacture of pipes.
WO-A-98/18554 discloses a process for the polymerization of alpha-olefins using an aluminophosphate support.
EP-A-0591968 discloses chromium catalyst compositions for use in polymerising olefins to produce polyethylene copolymers having good environmental stress crack resistance. Like EP-A-0647661, the chromium catalyst composition comprises at least two chromium catalyst systems having respective supports with different average pore radius. That difference is sufficient preferentially to introduce a non-ethylene comonomer into the higher molecular weight portion of the resulting copolymer. Like EP-A-0647661, EP-A-0591968 does not address the production of polyethylenes having crack resistance for use in the manufacture of pipes.
EP-A-0849293 discloses the production of polyethylene having a bimodal molecular weight distribution.
EP-A-0166157 discloses silicon and/or fluorine treated alumina for use in the production of olefin polymers having high density and ultra high molecular weight.
The present invention aims in one aspect to provide a process for producing polyethylene, in particular a polyethylene having a monomodal molecular weight distribution, having improved mechanical properties.
Accordingly, the present invention provides a process for producing a polyethylene resin having a monomodal molecular weight distribution in a single polymerisation reactor using a chromium-based catalyst system, by copolymerising ethylene and an alpha-olefinic-comonomer having from 3 to 8 carbon atoms, the polyethylene resin comprising a blend of a higher molecular weight fraction polymerised by a first chromium-based catalyst of the system and a lower molecular weight fraction polymerised by a second chromium-based catalyst of the system, wherein the comonomer incorporation in the higher molecular weight fraction is greater than the comonomer incorporation in the lower molecular weight fraction is greater than the comonomer incorporation in the lower molecular weight fraction whereby the higher and lower molecular weight fractions differ in density by from 0.01 to 0.03 g/cc.
The first and second catalysts may be differently treated portions of the same catalyst and physically blended before activation of the catalysts. Alternatively, the first and second catalysts may be different catalysts and physically blended before or after activation of the catalysts.
The present inventor has discovered surprisingly that by producing a polyethylene resin having a monomodal distribution using a single reactor but with two catalysts improved mechanical properties can be achieved by providing that the catalysts achieve greater comonomer incorporation in the higher molecular weight fraction than in the lower molecular weight fraction.